1. Technical Field
The present invention relates to nanotube-based electronic devices, including devices which incorporate organic molecules.
2. Background Art
The field of molecular electronics has become one of the most exciting technology areas in recent years. Molecular electronics devices are significantly smaller, more energy efficient and less expensive to manufacture than their silicon-based counterparts. They are regarded as one of the most promising technological alternatives to overcome the inherent scaling limits of silicon devices.
A basis for molecular electronics lies in organic molecules that are capable of conducting electricity and switching between on and off states as a result of external manipulations (in a similar manner as silicon-based transistors). One way to build such a molecular electronic device is to use an organic film as an active channel between the metallic source and drain electrodes. The molecular structure and a molecule's capability of packing in some form of ordered structure are crucial to facilitate electron transport through the channel. However, the selection of appropriate molecules has proven to be a great challenge.
An alternative way to build such a device is to bridge two ends of an individual molecule directly to the source and drain electrodes. This method does not require that the molecule form any ordered structure, and will result in a circuit with much smaller channel region with extraordinary properties. However, due to the constraints of traditional lithography, the gaps between the metal electrodes are usually large compared to the size of small organic molecules, making the bridging very difficult.
One feature of the latter kind of devices is very small contact areas between the conducting molecules and the electrodes. As a result, the electron transport at the junction points between the molecular wires and metal electrodes becomes significant in the circuit characteristics. However, bonding between organic molecules and metal electrodes is difficult to accomplish, and is notoriously ill-defined even when accomplished. For example, as reported in M. A. Reed et al., Science vol. 278, p. 252 (1997); A. Salomon et al., Adv. Mater. vol. 15, p. 1881 (2003), no methods have been identified to control the type of metal-molecule bonding in the most well-studied system involving thiolated molecules on Au contacts. Moreover, as reported in H. Basch et al., Nano Lett. vol. 5, p. 1668 (2005), even if more conductive contact chemistry is used, such as carbenes on transition metals and on metal carbides, molecular-scale metal electrodes are extremely difficult to fabricate and lack specific chemistry for molecular attachment at their ends. This ill-defined bonding may result in unpredictable transport properties of electrons through the devices.
Carbon nanotubes provide new alternatives in molecular electronics research. Carbon nanotubes are a unique carbon-based molecular structure, consisting of graphitic layers wrapped to cylinders, usually having an extremely high length/width ratio. Carbon nanotubes can have multi-walls on their cylindrical shells, or only a single atomic layer. The latter is referred to as Single Wall Carbon Nanotubes (“SWNTs”), which have narrower diameters (typically in the range of 1˜2 nm) and fewer defects on their molecular structure than their multi-walled counterparts. Depending on their chirality and diameters, SWNTs may be metallic or semiconducting. Due to their intriguing structure and unique electronic properties, SWNTs have become one of the most actively studied nanostructures in the past decade, and molecular electronic devices such as field effect transistors based on semiconducting SWNTs have been studied with increasing interest. For example, U.S. Patent Pub. No. 2004/0144972 to Dai et al., discloses a voltage controllable nanotube device where a gate electrode is capacitively coupled to a carbon nanotube via high-κ dielectric material.
The high aspect ratio of SWNTs makes them good candidates for constructing a molecular electronic device because metallic electrodes can be placed at a distance by traditional lithography methods. However, this benefit can also be a barrier for new generation nanometer-scale transistors. Reducing the width of active channels in these transistors is still a great challenge.
SWNTs have also been reported in sensing applications, such as high sensitivity gas detectors and glucose sensors. See S. Chopra et al., App. Phys. Lett. vol. 83, p. 2280 (2003); S. Chopra et al., App. Phys. Lett., vol. 80, p. 4632 (2002); P. W. Barone et al., Nat. Mat. vol. 4, p. 86 (2005). Physical affinity or chemical reactivity of SWNTs toward the molecules to be detected is the basis for these applications. However, since SWNTs have a large surface area and multitude of potential reaction centers, the specificity and sensitivity of the detection are still limited. Accordingly, a need remains for a technique for fabricating electronic devices from SWNTs with appropriate organic molecules.